1. Field of the Invention
The invention relates to a charging device for use in an image forming apparatus as well as the image forming apparatus.
2. Description of Related Art
Charging devices are employed, e.g., in electrophotographic image forming apparatuses and particularly in various kinds of recording devices such as electrophotographic copying machines, facsimile machines and printers, and electrophotographic display devices.
For example, in an electrophotographic recording device, a charging device charges a surface of an image carrying member to bear a uniform potential, and image exposure corresponding to an image to be formed is effected on a region thus charged. Thereby, an electrostatic latent image is formed, and then is developed with toner to form a toner image, which is then transferred onto a recording medium such as transfer paper, and is fixed thereto by heat and pressure.
Further, a charging device may be used as a transfer charger for transferring the toner image onto the recording medium, and may also be used as a separation charger for separating the recording medium from the image carrying member after the transfer of the toner image.
Corona discharging devices are often used as charging devices in the image forming apparatuses. Typically, the corona discharging device includes a discharging electrode, which extends along a charge target member, i.e., a member to be charged, a so-called “stabilizer plate” or a so-called “shield casing” accommodating the discharging electrode, and a grid arranged in an opening formed in the stabilizer plate and is opposed to the charge target member.
A high voltage is applied to the discharging electrode, and a grid voltage is applied to the grid. The grid controls an amount of electric charges applied to the charge target member.
The discharging electrode may be formed of, e.g., a discharging wire or an electrode having a saw-toothed discharging end, as is known.
The discharging electrode, stabilizer plate and grid of the corona discharging device may be made of stainless steel having high corrosion resistance for minimizing oxidation of these parts because the corona discharging device generates ozone.
According to the discharging device disclosed in Japanese Laid-Open Patent Publication No. H7-28299 (28299/1995), the discharging electrode is formed of a saw-toothed discharging electrode, which generates a smaller amount of ozone than the wire electrode, and at least the discharging end portion (saw-toothed portion) is formed of an electrically conductive material containing 8%–15% of nickel and 16%–20% of chromium, or furthermore 2%–3% of molybdenum, or is coated with a material (e.g., dielectric material such as ceramics) having an high electric resistance.
Also, Japanese Laid-Open Patent Publication No. H11-40316 (40316/1999) has disclosed a grid body, which is formed of a perforated plate of stainless steel, and is coated with gold.
However, components of a charging device in an image forming apparatus are oxidized after it is used many times even if the components are made of stainless steel. When at least one of the discharging electrode, stabilizer plate and grid is oxidized to a certain extent, the discharging device, which is configured, e.g., to charge uniformly a surface of an image carrying member, cannot charge various portions of the surface of the image carrying member to bear a uniform potential, which may cause image noises (irregularities in density of a halftone image, striped image noises or the like) and/or adhesion of carrier onto the image carrying member (if two-component developer containing the carrier and tone is used).
According to the structure disclosed in Japanese Laid-Open Patent Publication No. H7-28299, the saw-toothed discharging electrode is employed, and at least the discharging end portion (saw-toothed portion) thereof is formed of an electrically conductive material containing 8%–15% of nickel and 16%–20% of chromium, or furthermore 2%–3% of molybdenum, or is coated with a material (e.g., dielectric material such as ceramics) having an high electric resistance. However, this structure cannot sufficiently suppress the oxidation, and oxidation will considerably occur to cause image noises when used for a long term.
According to the structure disclosed in Japanese Laid-Open Patent Publication No. H11-40316, the charging device member is coated with gold. This structure is excessively expensive.